Quite a few of my 6.5" speakers are brittle and have cracked over the years and will be replaced this off season. The small tweeters work still work great. My question is can the tweeters be reused with a new 6.5" speaker or do both need to be replaced. If it can be reused, how should the wiring be run? Is it as simple as tying the tweeter in the same connection as the 6.5"? Sorry if I am using the wrong terminology or this is a dumb question, don't know too much about audio.

The music signal goes through a crossover first, which sends lower freq sounds to the woofer, and higher freq to the tweeter. Most crossovers are designed and built according to the frequency response and brand of the speaker. On my boat, the crossover is attached to the back of the 6.5", but yours might be different - it's generally a small 4"x4" box with speaker terminals on it.

The short answer is considering the small cost you'd save by trying to reuse the tweeters, I'd just go ahead and buy all new. Most component speakers are sold as sets anyway, so you'd probably have a hard time finding just the 6.5" part without the tweeter/crossover.

If you are buying replacement woofers, providing they are still available, for those component sets, then yes, you can just reuse the tweeter and cross-over. If you are going to purchase a new set of components, then use all the new equipment and not reuse any of the old. If you are going with a standard coaxial as a replacement, then you would not want to wire the new speaker into the end of the existing speaker wire, where the old woofer was. You would want to take the tweeter and its leftover cross-over out of the circuit and use the speaker wire that comes directly from the amp chnl. With a coaxial that has its own grill mounted tweeter, you would not need or want that old tweeter that 6" away anyway.